Sorry, yeah, it is intended that all levels are beatable. To upload a level you must prove that it is beatable by clearing it from the beginning without dying, and then clearing it again from each checkpoint (if there are any) to prove that it can also be cleared from any checkpoints.
Hacked levels have existed that cannot be cleared, but they can be reported and Nintendo takes them down. They should all be taken down by now, but in any case if it’s obviously impossible (the goal is completely blocked by impenetrable walls) Team 0% marks them as hacked on their spreadsheet in addition to reporting them, so they wouldn’t count.
In this game you can download levels and see the full level in the editor, so it isn’t possible to make a level that is “practically” unclearable using hidden information. Any things like hidden keys, “passcode” sections (where you need to hit blocks a certain number of times in order to manipulate things hidden off screen), etc. are trivially defeated by viewing the level in the editor.